Method and system for the purification of exhaust gas from an internal combustion engine

ABSTRACT

The invention provides a method and system for the purification of exhaust gas from an internal combustion engine, comprising a filter and a SCR catalyst. The filter is periodically regenerated increasing the temperature of the exhaust gas up to 850° C. and the water vapour content up to 100% by volume. The SCR catalyst comprises a hydrothermally microporous stable zeolite and/or zeotype having the AEI type framework and being promoted with copper.

The present invention relates to after treatment of exhaust gas from aninternal combustion engine in terms of removal or reduction of harmfulcompounds. More particularly, the invention focus on removal ofparticulate matter and reduction of nitrogen oxides in engine exhaustfrom lean burn internal combustion engines, and in particular dieselengines.

Lean burn engines are known to be energy efficient, but have thedisadvantage of forming particulate matter and nitrogen oxides, whichmust be removed or at least reduced in the engine exhaust.

To prevent environmental pollution and to fulfil several governmentalrequirements, modern diesel engines are provided with an exhaust gascleaning system comprising in series an oxidation catalyst for theremoval of volatile organic compounds, a particulate filter for theremoval of particulate matter and a catalyst being active in theselective reduction of nitrogen oxides (NOx).

It is also known to integrate the SCR catalyst into the particulatefilter.

Selective catalytic reduction of NOx in exhaust gas is usuallyaccomplished by reaction with ammonia introduced as such or as aprecursor thereof, which is injected into the exhaust gas upstream ofthe SCR catalyst for the selective reduction of nitrogen oxides, mainlynitrogen dioxide and nitrogen monoxide (NOx), to nitrogen.

For this purpose numerous catalyst compositions are disclosed in theliterature.

Lately, zeolites promoted with copper or iron, have found greatinterest, particularly for use in automotive application.

Copper containing zeolite catalysts for NH₃-SCR applications have shownhigh activity at low temperature. However, in certain applications thecatalyst can be exposed to high temperature excursions in exhaust gases.Furthermore the exhaust gas contains high concentrations of water vapourfrom the combustion engine, which can deteriorate the zeolite catalystperformance. The hydrothermal stability is often an issue for Cu-basedzeolites catalysts as one possible catalyst deactivation mechanism isthe degradation of the zeolite framework due to its instability towardshydrothermal conditions, which is furthermore enhanced by the presenceof copper.

Deactivation of copper containing zeolite catalysts in NH₃-SCRapplications is typically caused by degradation of the zeolite frameworkdue to its instability towards hydrothermal conditions, which isfurthermore enhanced by the presence of copper. However the stability isespecially important for automotive applications in which the catalystwill experience high temperature excursions in an exhaust streamcontaining water.

Deactivation of the catalyst is in particular a problem in exhaust gascleaning systems provided with a particulate filter, which mustperiodically be actively regenerated in order to prevent build up ofpressure over the soot laden filter.

Active regeneration is performed by burning of captured soot. Theregeneration can be initiated by injection of fuel into the exhaust gasupstream the oxidation catalyst or by electrical heating of theparticulate filter.

During the active regeneration exhaust gas temperature at outlet of thefilter can reach more than 850° C. and a content of water vapour morethan 15% and up to 100% for periods of time between 10 and 15 minutesdepending on the amount of soot captured in the filter.

It is the general object of the invention to provide a method for theremoval of harmful compounds lean burn internal combustion engines, suchas particulate matter by means of a particulate filter and nitrogenoxides by selective catalytic reduction of nitrogen oxides in contactwith catalyst being hydrothermally stable when exposed to hightemperatures and water vapour concentration during active regenerationof the particulate filter.

We have found that the object of the invention can be achieved by usinga zeolite or zeotype having hydrothermally stable AEI type framework, inwhich the structure is preserved under hydrothermal aging conditionseven when copper is present in the zeolite or zeotype.

Pursuant to the above finding, this invention provides a method for thepurification of exhaust gas from an internal combustion engine,comprising

reducing the content of soot in the exhaust gas by passing the gasthrough a particulate filter;

subsequently reducing the content of nitrogen oxides in presence ofammonia or a precursor thereof by contact with a catalyst being activein NH3-SCR;

periodically regenerating the filter by burning of soot captured in thefilter and thereby increasing temperature of the exhaust gas up to 850°C. and water vapour content up to 100% by volume; and

passing the exhaust gas from the filter through the catalyst during theregeneration of the filter, wherein the catalyst comprises ahydrothermally stable zeolite and/or zeotype having an AEI typeframework and copper incorporated in the framework.

“Hydrothermally stable” means that the zeolite and zeotype catalyst havethe ability to retain at least 80 to 90% of initial surface area and 80to 90% microporous volume after exposure to temperatures of at least600° C. and a water vapour content up to 100 volume % for 13 hours, andat least 30 to 40% of initial surface area and micropore volume afterexposure to temperatures of at least 750° C. and a water vapour contentup to 100 volume % for 13 hours.

Preferably, the hydrothermally stable zeolite or zeotype with an AEItype framework has an atomic ratio of silicon to aluminium between 5 and50 for the zeolite or between 0.02 and 0.5 for the zeotype.

The most preferred zeolite or zeotype catalysts for use in the inventionare zeolite SSZ-39 and zeotype SAPO-18 both having the “AEI” frameworkstructures, in which copper is introduced by impregnation, liquid ionexchange or solid ion exchange.

The atomic copper to aluminium ratio is preferred to be between about0.01 and about 1 for the zeolite. For the zeotype the preferred atomiccopper to silicon ratio is correspondingly between 0.01 and about 1.

By means of the above catalysts employed in the invention, 80% of theinitial NOx reduction is maintained at 250° C. after aging at 750° C. ascompared to 20% for a Cu-CHA catalyst.

Thus, in an embodiment of the invention, 80% of the initial reduction ofnitrogen oxides at 250° C. is maintained after the catalyst has beenexposed to a temperature of 750° C. and a water vapour content of 100%in the exhaust gas for 13 hours.

The invention provides in addition an exhaust gas cleaning system,comprising an active regenerable particulate filter and an SCR catalystcomprising a hydrothermally microporous stable zeolite and/or zeotypehaving the AEI type framework and being promoted with copper.

In an embodiment of the exhaust gas cleaning system according to theinvention, the SCR catalyst is integrated into the particulate filter.

In further an embodiment, the atomic copper to aluminium ratio isbetween about 0.01 and about 1 for the zeolite and the atomic copper tosilicon ratio is between 0.01 and about 1 for the zeotype.

In still an embodiment, the atomic ratio of silicon to aluminium in theSCR catalyst is between 5 and 50 for the zeolite and between 0.02 and0.5 for the zeotype.

In a further embodiment, the SCR catalyst retains 80% of the initialreduction of nitrogen oxides at 250° C. after the catalyst has beenexposed to a temperature of 750° C. and a water vapour content of 100%in the exhaust gas for 13 hours.

In a further embodiment, the SCR catalyst retains 80 to 90% of theinitial microporosity after aging at 600° C., and 30 to 40% of theinitial microporosity after aging at 750° C.

In still an embodiment, the SCR catalyst is an aluminosilicate zeoliteSSZ-39 and/or silicoaluminum phosphate SAPO-18.

In the above embodiments, the SCR catalyst can be deposited on amonolithic support structure.

The Cu-SSZ-39 catalyst system has shown an improved performance comparedto the typical “state-of-the-art” Cu-SSZ-13 when similar Si/Al ratiosare compared.

EXAMPLE 1 Cu-SSZ-39 Catalyst Preparation

The zeolite SSZ-39 with the framework type code AEI was synthesized in asimilar way as given in U.S. Pat. No. 5,958,370 using1,1,3,5-tetramethylpiperidinium as the organic template. A gel with thefollowing composition: 30 Si:1.0 Al:0.51 NaOH:5.1 OSDA:600 H₂O, wasautoclaved at 135° C. for 7 days, the product filtered, washed withwater, dried and calcined in air. The final SSZ-39 had a Si/Al=9.1measured by ICP-AES.

To obtain the Cu-SSZ-39 the calcined zeolite was ion exchanged withCu(CH₃COO)₂ to obtain the final catalyst with a Cu/Al=0.52 aftercalcination.

The powder X-ray diffraction (PXRD) pattern of Cu-SSZ-39 aftercalcination is shown in FIG. 1.

EXAMPLE 2 Catalytic Testing

The activity of the samples for the selective catalytic reduction ofNO_(x) was tested in a fixed bed reactor to simulate an engine exhauststream using a total flow rate of 300 mL/min consisting of 500 ppm NO,533 ppm NH₃, 7% O₂, 5% H₂O in N₂ in which 40 mg catalyst was tested.

The NO_(x) present in the outlet gases from the reactor were analyzedcontinuously and the conversion is shown in FIG. 2.

EXAMPLE 3 Test of Hydrothermal Durability

In order to test the hydrothermal stability of the zeolites, steamingtreatments were done to the samples. They were exposed to a water feed(2.2 mL/min) at 600 or 750° C. during 13 hours in a conventional ovenand afterwards tested similarly to Example 2.

The catalytic results can also be seen in FIG. 2. The samples thatunderwent a hydrothermal treatment have been marked with 600 or 700° C.,depending on the temperature used during the hydrothermal treatment.

Additional characterization has also been performed to all treatedsamples. PXRD patterns after hydrothermal treatments are shown in FIG.1, and BET surface areas, micropore areas, and micropore volumes oftreated samples are summarized in Table 1 below.

EXAMPLE 4 Comparative Example with Cu-CHA (Cu-SSZ-13)

A Cu-CHA zeolite was prepared from a gel with the molar composition:SiO₂: 0.033 Al₂O₃: 0.50 OSDA:0.50 HF:3 H₂O, where the OSDA isN,N,N-trimethyl-1-adamantamonium hydroxide.

The gel was autoclaved at 150° C. for 3 days under tumbling to give afinal zeolite product with a Si/Al=12.7 after washing, drying andcalcination.

To obtain the Cu-CHA the calcined zeolite was ion exchanged withCu(CH₃COO)₂ to obtain the final catalyst with a Cu/Al=0.54.

The powder X-ray diffraction (PXRD) pattern of Cu-CHA after calcinationis shown in FIG. 1.

This catalyst was also tested according to example 2, and thehydrothermal durability evaluated similarly to example 3. The catalyticresults are summarized in FIG. 2 of the drawings. PXRD patterns oftreated-CHA samples are shown in FIG. 1, and textural properties (BETsurface area, micropore volume, and micropore area) are summarized onTable 1.

TABLE 1 Volume BET surface Micropore micropore Sample area (m²/g) area(m²/g) (cm³/g) SSZ-39_Calc 571 568 0.28 SSZ-39_600° C. 554 551 0.28SSZ-39_750° C. 565 563 0.28 Cu-SSZ-39_600° C. 465 463 0.24Cu-SSZ-39_750° C. 158 152 0.09 CHA_calc 675 637 0.32 CHA_600° C. 687 6450.32 CHA_750° C. 674 623 0.31 Cu-CHA_600° C. 633 585 0.29 Cu-CHA_750° C.50 35 0.02

EXAMPLE 5 Cu-SAPO-18

Silicoaluminophosphate SAPO-18 with the framework type code

AEI was synthesized according to [J. Chen, J. M. Thomas, P. A. Wright,R. P. Townsend, Catal. Lett. 28 (1994) [24]-248] and impregnated with 2wt. % Cu. The final Cu-SAPO-18 catalyst was hydrothermally treated in10% H₂O and 10% O₂ at 750° C. and tested under the same conditions asgiven in Example 2. The results are shown in FIG. 2 of the drawings.

1. Method for the purification of exhaust gas from an internalcombustion engine, comprising reducing the content of soot in theexhaust gas by passing the gas through a filter; subsequently reducingthe content of nitrogen oxides in presence of ammonia or a precursorthereof in contact with a catalyst being active in NH3-SCR; periodicallyregenerating the filter by burning of soot captured in the filter andthereby increasing temperature of the exhaust gas up to 850° C. andwater vapour content up to 100% by volume; and passing the exhaust gasfrom the filter through the catalyst during the regeneration of thefilter, wherein the catalyst comprises a hydrothermally microporousstable zeolite and/or zeotype having the AEI type framework and beingpromoted with copper.
 2. The method of claim 1, wherein the atomiccopper to aluminium ratio is between about 0.01 and about 1 for thezeolite and the atomic copper to silicon ratio is between 0.01 and about1 for the zeotype.
 3. The method of claim 1, wherein the atomic ratio ofsilicon to aluminium is between 5 and 50 for the zeolite and between0.02 and 0.5 for the zeotype.
 4. The method of claim 1, wherein 80% ofthe initial reduction of nitrogen oxides at 250° C. is maintained afterthe catalyst has been exposed to a temperature of 750° C. and a watervapour content of 100% in the exhaust gas for 13 hours.
 5. The method ofclaim 1, wherein at least 80 to 90% of the initial microporosity ismaintained after aging at 600° C., and at least 30 to 40% is maintainedafter aging at 750° C.
 6. The method of claim 1, wherein the catalyst isan aluminosilicate zeolite SSZ-39 and/or silicoaluminum phosphateSAPO-18.
 7. An exhaust gas cleaning system, comprising an activeregenerable particulate filter and an SCR catalyst comprising ahydrothermally microporous stable zeolite and/or zeotype having the AEItype framework and being promoted with copper.
 8. The exhaust gascleaning system of claim 7, wherein the SCR catalyst is integrated intothe particulate filter.
 9. The exhaust gas cleaning system of claim 7,wherein the atomic copper to aluminium ratio is between about 0.01 andabout 1 for the zeolite and the atomic copper to silicon ratio isbetween 0.01 and about 1 for the zeotype.
 10. The exhaust gas cleaningsystem of claim 7, wherein the atomic ratio of silicon to aluminium inthe SCR catalyst is between 5 and 50 for the zeolite and between 0.02and 0.5 for the zeotype.
 11. The exhaust gas cleaning system of claim 7,wherein the SCR catalyst retains 800 of the initial reduction ofnitrogen oxides at 250° C. after the catalyst has been exposed to atemperature of 750° C. and a water vapour content of 100% in the exhaustgas for 13 hours.
 12. The exhaust gas cleaning system of claim 7,wherein the SCR catalyst retains at least 80 to 90% of the initialmicroporosity after aging at 600° C., and at least 30 to 40% of theinitial microporosity after aging at 750° C.
 13. The exhaust gascleaning system of claim 7, wherein the catalyst is an aluminosilicatezeolite SSZ-39 and/or silicoaluminum phosphate SAPO-18.
 14. The exhaustgas cleaning system of claim 7, wherein the SCR catalyst is deposited ona monolithic support structure.